list.rb: new syntax for multiple + versions flags

Library/Homebrew/cmd/list.rb

@@ -31,15 +31,10 @@ module Homebrew
       switch "--pinned"
       switch "--versions"
       switch "--full-name"
-      switch "--multiple"
+      switch "--multiple", required_for: "--versions"
       switch :verbose
     end
 
-    if @args.multiple? && !@args.versions?
-      opoo "--multiple requires --versions"
-      @args.send("versions?=", true)
-    end
-
     # Use of exec means we don't explicitly exit
     list_unbrewed if @args.unbrewed?